Envoy currently doesn't support being used as a forward proxy. Forward proxy support would mean that any standard web browser could set an envoy as their proxy and send requests into a service mesh. I've hashed out some initial support in an internal build. I propose to add forward proxy support to envoy in 3 stages. I think that what roughly needs to happen is:

1. relax the assumptions that envoy makes about differences in url handling and connection management. proxy keep alive vs keep alive, absolute uri in method line
2. Add a notion of a 'default cluster' or similar fallthrough resolution. In my environment its sufficient to forward any traffic that isn't destined for something defined to border proxy, but i suspect more full fledged support for locating 'external' destinations would be useful.
3. Add CONNECT support.